Generally the invention relates to the field known as thin film processing through sputter techniques. More specifically, the invention focuses on the undesirable aspect of electrical shorts during the sputter process, presenting both methods and apparatuses for removing such electrical shorts upon their occurrence.
The sputtering process in general is well known in the art. It was apparently first reported in 1852 by Sir William Robert Groves. In 1921 Joseph John Thompson initially named the process "spluttering"; later the "1" was dropped and the name for the process became "sputtering." This term is meant to describe a process whereby atoms of a material are mechanically freed from a surface through a momentum transfer. The atoms then dissipate to cause a thin film on or interact with a surface or substrate. Although the sputtering process has been known for some time, in recent years applications of the process have grown dramatically and have been subject to refinement and development with technological advances. To a significant degree the increase in focus on the sputtering process has been due to the growth of the semiconductor industry which has increased the focus on thin film processes in recent years and has made available more sophisticated equipment to practice the process.
To properly understand the problem addressed through the present invention, it is necessary to generally understand the operation of the sputter process. An excellent discussion of the process is contained in the textbook Glow Discharge Processes by Brian Chapman published in 1980 by John Wiley & Sons, Inc. As relevant to the present invention, the sputter process is a process whereby the surface of an item is coated (deposition), removed (etched), or whereby the surface of an item is conditioned. In basic form, a sputtering system involves a power supply which ionizes a gas. This ionized gas or plasma then accelerates to a target which contains the material that will become the coating. When the ions strike the target, atoms of the target are released through momentum transfer. These atoms then dissipate and some eventually contact a substrate and become a coating on that substrate. Although it is a goal to achieve as uniform a process as possible, inherent noise and anomalies in both the plasma generation and the material of the target can cause fluctuations in the process. These fluctuations can result in the release of particles or flakes from the target rather than the individual atoms desired. In addition--and perhaps to a more significant degree--the coating itself tends to peel off of all surfaces in the chamber. When it peels off of any surface, flakes are created which fall or lodge across the power supply. Also, the coating can build up in undesirable places. This results in the same effects as a flake and is thus referred to synonymously as such. In particular target geometries, occasionally one of these flakes occurs across the power supply causing an electrical short. Such a short is not only undesirable but they are very difficult to predict because the entire environment is very dynamic.
The significance of these flakes for the particular target geometries involved is a function of two factors: the energy capacity of the power supply and the particular target material involved. In prior years it was common to utilize power supplies which rectified relatively low frequency line sources and which had high energy capacities relative to the amount of energy necessary to melt the types of flakes which are the focus of the present invention. In recent years it has become understood by those skilled in the art that higher frequency, lower capacity power supplies provide better process control for reasons unrelated to the occurrence of flakes. Although the older, high energy capacity power supplies easily release sufficient energy into some flakes to not just melt, but to vaporize the flake, larger flakes exist which are not melted during normal processing. When the more sophisticated, lower energy capacity power supplies are utilized, however, the flake problem becomes more acute. Not only are the larger flakes not melted, but the smaller flakes now become a problem as well. These lower energy storage power supplies simply react too quickly and have insufficient stored energy to melt even the smaller flakes which occur. Prior to the present invention, when a flake did occur for these more sophisticated power supplies, an over current condition would automatically shut down the power supply. Those skilled in the art then would manually shut down the entire sputter system and would physically remove the flake from the target surface. This not only necessitated long time delays but it also caused an unevenness in the treatment of the substrate. The present invention addresses this problem as it relates to both the older, higher energy storage power supplies and as it relates to the more sophisticated, lower energy storage power supplies. Since the problem is more acute in the lower energy storage power supplies, however they are the focus of this disclosure.
As mentioned, another factor involved in the occurrence of flakes is the nature of the particular target involved. Although almost all targets are commercially manufactured to minimize the possibility of an occurrence of a flake or impurity being released, some materials are more prone to this than others. For instance the invention in U.S. Pat. No. 4,610,775 to Phifer is directed to the occurrence of flakes when coating nuclear fuel pellets with a thick layer of zirconium diboride (ZrB.sub.2). Apparently this material has a tendency to produce flakes. The Phifer disclosure patents the solution of providing a separate AC power supply, which provides 150 to 200 amps at 60 volts AC to melt those flakes. In sharp contrast, the present invention provides a simpler, less expensive solution which utilizes the existing power supply to melt the flakes. While the problem of electrical shorts caused by flakes has been known, the fact that the flakes shorted the power supply utilized in normal operation led those skilled in the art away from utilizing it as a solution to the problem. Since the flake shut down the power supply, it was felt that an external solution was required. Also in sharp contrast, the present invention provides only sufficiently enough energy to melt the flake, rather than to vaporize it. This avoids uncontrolled sputtering and provides a more uniform end result than solutions which may vaporize the flake. This is because "vaporizing" the flake results in it exploding and splattering material inside the system.
Similarly, inventions such as discussed in U.S. Pat. Nos. 3,544,913 and 3,546,606 to Anderson involve a totally different environment. As mentioned, those inventions relate to the field of electron beam processing in which an arc discharge is "starved" before development by quickly limiting the current which creates the electron beam. Because those inventions involve arcs rather than flakes, the clearing or removal process is fundamentally different. The electrical short is not "starved" in the present invention, rather it is melted by driving more current through it.